In various optical communication applications, it is necessary to amplify an optically transmitted data signal. One method of amplifying optical signals is through the use of a doped fiber amplifier, such as an erbium doped fiber amplifier (EDFA). Fiber amplifiers of this type receive pump light of a first wavelength and transfer its energy to an input optical signal of a second wavelength. For example, in an EDFA, erbium doped fibers absorb the energy in applied pump light having a wavelength in the range of 980 nm. The presence of a 1550 nm input signal in the erbium fiber stimulates the fiber to emit the stored energy at the input signal 1550 nm wavelength, thus amplifying the input signal. Although various mechanisms are suitable for coupling the 980 nm pump light into the erbium doped fiber (EDF), the pump light is conventionally supplied by a laser and combined with the 1550 nm signal in the EDFA using a wavelength division multiplexer (WDM). Other dopants, which are typically rare earth elements, may be used to produce fiber amplifiers operating with other pump and signal wavelengths.
In conventional EDFA systems, the 1550 nm input data signal is supplied to the WDM from a 1550 nm single-mode optical fiber. In other words, the fiber has a core region small enough to prevent the data signal from propagating in more than one spatial mode. The pump energy is coupled to the WDM by a second fiber. Conventionally, this fiber is single mode at the shorter pump wavelength (980 nm) and therefore has a narrower core than the first fiber. The 1550 nm and 980 nm light sources are combined by directing them at an appropriate multiplexing device, such as a dichroic mirror or a prism assembly.
The combination of pump and input signal energy is directed into a third (output) optical fiber. The core of the third optical fiber is conventionally chosen to be single mode for both the 980 nm pump energy and the 1550 nm data signal. The output fiber may be erbium doped or may be coupled to a separate erbium doped fiber. The erbium doped fiber absorbs the 980 nm pump light and amplifies the 1550 nm input data signal.
As is apparent, one characteristic of a conventional EDFA is that the optical fibers which carry the pump light are single mode fibers for the shorter pump wavelength. Because of the smaller core size required for single-mode propagation at the shorter wavelength, aligning optical components to receive and/or direct a light signal into the end of the fiber requires a higher degree of precision. Because of this, conventional EDFAs must be manufactured with tighter design tolerances than if a fiber having a wider core were used. In addition, because of the demanding precision required to align the optical components, a relatively large number of devices must be rejected during the manufacturing process. Finally, while a single mode fiber is used to carry the pump light to the WDM, fiber that is single mode at the pump wavelength is not conventionally used on WDM inputs or outputs, which do not carry the pump light. Because different fiber types are used for input to and output from the WDM, a mode mismatch situation occurs, and the accompanying decrease in coupling efficiency results in a noticeable degree of signal loss.